The need to create and control time delay in a signal arises in many different applications. One such an application is in a linear power amplifier (LPA). In a typical LPA, a distortion signal is time delayed before being combined with a composite signal. The distortion signal and the composite signal are combined to cancel the distortion component from the composite signal; as a result, a signal that is essentially free of distortion is produced. Creating accurate and precise time delay in the distortion signal is essential for efficient cancellation of the distortion component from the composite signal.
Referring to FIG. 1, a simplified block diagram of an LPA 100 is shown. LPA 100 receives at an input 121 a signal 101 to be amplified. A splitter 102 splits signal 101 into two essentially identical signals 103 and 104 at outputs 132 and 133 respectively. Signal 103 is coupled to an input 122 of an amplifier 105. A composite signal 107 outputs at an output 124 of amplifier 105. Composite signal 107 essentially composed of signal 103 at an amplified level and distortion signal produced by amplifier 105. Signal 104 is coupled to an input 123 of a time delay block 106. Time delay block 106 time delays signal 104 and produces a time delayed signal 108 at an output 125.
A distortion cancellation loop 190 in LPA 100 cancels the distortion signal of composite signal 107 before outputting an amplified signal 118 at an output 191. Amplified signal 118 is an amplified version of signal 101 but essentially free of distortion. There may be more than one distortion cancellation loop in LPA 100, however, for simplicity of explanation, only one is shown. A portion of composite signal 107 is coupled via coupler 109 as signal 140. Signal 140 passes via a connection 120 to a coupler 110 to be coupled with time delayed signal 108. The amount of time delay in time delay block 106 is proportional to an amount of time delay which signal 103 experiences by propagating through amplifier 105 and coupler 109 before reaching coupler 110. The coupling effect of signals 108 and 140 is that components of signal 103 present in signal 140 are essentially canceled. Signal 140 essentially has all the components of signal 107. As a result, when signal 108 couples with signal 140, a distortion signal 112 is produced which is essentially composed of the distortion signal present in composite signal 107.
Distortion signal 112 passes to an input 127 of an amplifier 114 to produce amplified distortion signal 116 at an output 128. Amplifier 114 amplifies signal 112 without essentially adding to distortion level of signal 112. Amplifier 114 in addition to amplifying signal 112 adjusts the phase of signal 112 and time delays signal 112 before producing signal 116. After composite signal 107 passes through coupler 109, it inputs as signal 111 at an input 126 of a time delay block 113. Time delay block 113 time delays signal 111 essentially equal to time delay that signal 112 experiences by propagating through amplifier 114, and produces signal 115 at output 131. Signals 115 and 116 are combined in combiner 117 to produce signal 118. Since signal 116 composed essentially of distortion signal in signal 115, combining them in combiner 117 essentially cancels the distortion signal from signal 115 and produces signal 118. Signal 118 is as a result free of distortion. Time delay block 106 may include other functions, not shown, such as signal amplitude and phase adjustments; therefore, time delay block 106 may be referred to as an equalization block incorporating time delay, phase and amplitude adjustments.
The cancellation of distortion components from signal 115 in combiner 117 is very critical for operation of LPA 100. Although other time delays are performed in LPA 100, the time delay performed in amplifier 114 is probably the most critical. The time delay and phase change occurring in amplifier 114 should be done with precision. Time delay of signal in amplifier 114 may be performed by adjusting electrical length of a coaxial cable used for propagating the signal. Use of coaxial cable for adjusting time delay has at least two negative consequences. The first consequence is effect of temperature and age on the actual time delay. This problem may only be corrected by periodic inspections and field adjustments of the amplifier long after it has been installed in the filed. Field maintenance may become a major problem. The second consequence is that a change of electrical length of the coaxial cable for adjusting the time delay also effects the signal phase. Therefore, at least an additional phase adjustment of the signal is needed with any change in the electrical length of the coaxial cable.
Therefore, there is a need for an efficient apparatus of variable time delay network and method for controlling the variable time delay network for time delaying a signal without requiring a periodic field inspection, and which additionally does not effect phase of the signal.